My second pixel art piece and another challenge entry.
Yeah, I can see, thanks for the advice :)
Good start, but I'm sure you could take this a lot further.
A few possible directions:
The tree is the main element, needs more detail, esp foliage (or branches instead).
Grass could use a little more stuff in it too, unless backlighting makes it a dark silhouette, which it is not (because darker shadow).
Shading isn't consistent: most of the tree should be in deep shadow because, again, backlighting (no reason the cast shadow would be darker than the unlit parts of the tree itself).
The sea and sky have some kind of depth conflict: while this is not always true, sea will often be darker than sky. Imagine that your image is upside down: see what I mean?
But the main point is the way you use colors, as if coloring a map so that no 2 countries are the same color! As a result the composition looks 'strident'. Try to recycle colors throughout the piece; it's ok to have different color atmospheres in different image zones but some colors can be used to link or unify those zones. Have a look at this tuto, esp the knight and lion piece: see how each are clearly different colors, yet some of the colors are in both?